Photosynthetic response to nitrogen starvation and high light in Haematococcus pluvialis

Astaxanthin is a carotenoid mainly produced by microalgae upon exposure to stress conditions: this pigment has anti-oxidant, anti-inflammatory and anti-cancer capacity and it is widely used as pigmentation agent in different industrial sectors. Abiotic stresses such as exposure to high irradiances and/or nitrogen starvation are commonly used to induce astaxanthin biosynthesis in freshwater green alga Haematococcus pluvialis. In this work high light and nitrogen deprivation were applied as single or combined stresses in order to investigate their influence on the photosynthetic properties of H. pluvialis cultures. The results reported here demonstrate that nitrogen starvation inhibits chlorophyll biosynthesis and favors chlorophyll b degradation, chlororespiration and cyclic electron transport, while cells grown in high light are characterized by a higher destabilization of PSII. The combination of high light and nitrogen deprivation induced the highest astaxanthin production and also the fastest photoprotective response which cooperatively prevented Photosystem II from the damage observed in high light stress and nitrogen supplemented medium. In these conditions inhibition of astaxanthin accumulation leads to a reduced cell size but does not induce a higher photosensitivity of photosynthetic machinery.